Temperature sensors are widely used in instrumentation and control systems, e.g., to monitor thermal conditions. There are a variety of types of temperature sensors, such as thermistors, resistance temperature detectors (RTDs), thermocouples and Silicon PN junction sensors. An advantage of a Silicon PN junction sensor is that it is easily integrated with Silicon circuitry that process sensor signals, such as an analog to digital converter (ADC) and voltage to temperature converter, which improves the accuracy and cost of a temperature detector system. A bandgap reference temperature sensor is a type of Silicon PN junction sensor.
FIG. 1 illustrates a conventional bandgap reference temperature detection system. Conventional temperature detection system 100, which generates an output signal, conventional temp, comprises conventional stimulator 110, conventional temp sensor 120, conventional sigma-delta ADC 130 and conventional voltage to temperature converter 160. Conventional stimulator 110 comprises first and second p-channel Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) M1P, M2P. Conventional temp sensor 120 comprises PNP Bipolar Junction Transistors (BJTs) Q1P, Q2P, first and second resistors R1, R2 and op amp 125. Conventional sigma-delta (ΣΔ) ADC (analog to digital converter) 130 comprises conventional sigma delta modulator 140 and conventional digital filter 150.
In conventional stimulator 110, source terminals of first and second FETs M1P, M2P are coupled to power supply VDD. Gate terminals of first and second FETs M1P, M2P are coupled to and controlled by the output of op amp 125. The drain terminal of first FET M1P is coupled to one terminal of second resistor R2. The drain terminal of second FET M2P is coupled to a positive input of op amp 125 and a terminal of first resistor R1. A second terminal of second resistor is coupled to an emitter terminal of first BJT Q1P. A second terminal of first resistor R1 is coupled to an emitter terminal of second BJT Q2P. First and second BJTs Q1P, Q2P are diode-connected. The base and collector terminals of each of first and second BJTs Q1P, Q2P are coupled to ground. A bandgap reference voltage VREF is drawn from the drain terminal of first FET M1P. A differential change in voltage across first resistor R1 is coupled to a differential input of conventional sigma delta modulator 140. A digital output of conventional sigma delta modulator 140 is coupled to the input of conventional digital filter 150. Conventional digital filter 150 removes high frequency noise caused by sigma delta modulator 140. The digital output of conventional digital filter 150 is provided to conventional voltage to temperature converter 160, which generates conventional temperature CTemp.
First and second FETs M1P, M2P are designed to be the same and to generate the same current in a current mirror. First and second BJTs Q1P, Q2P are designed to be different. The emitter area of second BJT Q2P is m times larger than the emitter area of first BJT Q1P. The current through first and second BJTs Q1P, Q2P is the same, but the current density in each of them is m times different. Since the emitter of second BJT Q2P is m times larger than the emitter of first BJT Q1P, the current density in second BJT Q2P is m times lower than the current density in first BJT Q1P. Op amp 125 causes the voltage at the emitter of first BJT Q1P, i.e., VBE1, to be the same as the voltage at the emitter of second BJT Q2P, i.e., VBE2, plus the voltage across first resistor R1, i.e., VR1, which renders VR1 equivalent to VBE1−VBE2 or ΔVBE. This voltage is proportional to absolute temperature (PTAT).
Given that M1P and M2P are designed to be the same, given that the emitter area of second BJT Q2P is m times larger than the emitter area of first BJT Q1P and given that the base of first and second BJTs Q1P, Q2P are coupled to ground, the base to emitter voltages VBE1, VBE2 for respective first and second BJTs Q1P, Q2P are given by equations 1.1 and 1.2:
                              V                      BE            ⁢                                                  ⁢            1                          =                              kT            q                    ⁢                      ln            ⁡                          (                                                I                                      C                    ⁢                                                                                  ⁢                    1                                                                    I                                      s                    ⁢                                                                                  ⁢                    1                                                              )                                                          Equation        ⁢                                  ⁢        1.1                                          V                      BE            ⁢                                                  ⁢            2                          =                              kT            q                    ⁢                      ln            ⁡                          (                                                I                                      C                    ⁢                                                                                  ⁢                    2                                                                    I                                      s                    ⁢                                                                                  ⁢                    2                                                              )                                                          Equation        ⁢                                  ⁢        1.2            where k is Boltzmann's constant, T is the temperature in Kelvins, q is the charge of an electron, IC1 is the current through the collector of first BJT Q1P, IC2 is the current through the collector of second BJT Q2P, IS1 is the saturation current of first BJT Q1P, IS2 is the saturation current of second BJT Q2P and ln is the natural logarithm function. Since the emitter area of Q2P is m times of Q1P, IS2 is m times of IS1.
Since the current through collector terminals of first and second BJT Q1P, Q2P is the same, i.e., IC1=IC2, the difference ΔVBE between base to emitter voltages VBE1 and VBE2, which is the voltage across first resistor R1, is given by equation 1.3:
                              Δ          ⁢                                          ⁢                      V            BE                          =                                            V                              BE                ⁢                                                                  ⁢                1                                      -                          V                              BE                ⁢                                                                  ⁢                2                                              =                                                    kT                q                            ⁢                              ln                ⁡                                  (                                                                                    I                                                  C                          ⁢                                                                                                          ⁢                          1                                                                    ⁢                                              I                                                  S                          ⁢                                                                                                          ⁢                          2                                                                                                                                    I                                                  S                          ⁢                                                                                                          ⁢                          1                                                                    ⁢                                              I                                                  C                          ⁢                                                                                                          ⁢                          2                                                                                                      )                                                      =                                          k                q                            ⁢                                                ln                  ⁡                                      (                    m                    )                                                  ·                T                                                                        Equation        ⁢                                  ⁢        1.3            The voltage across the first resistor R1 is proportional to absolute temperature (PTAT). Accordingly, the junction voltage difference ΔVBE is referred to as the PTAT voltage. If the current density m were designed to be 8, at room temperature of 300 Kelvins, the difference ΔVBE between base to emitter voltages VBE1 and VBE2 is approximately 53.7 mV according to Equation 1.3.
Bandgap reference voltage VREF can be determined relative to the junction voltage difference ΔVBE between base to emitter voltages VBE1 and VBE2. First and second FETs M1P, M2P are the same size, have the same gate to source terminal voltage VGS1, VGS2 and have the same drain current ID1, ID2. Second drain current ID2 is equivalent to the difference ΔVBE between base to emitter voltages VBE1 and VBE2 divided by first resistor R1. Accordingly, bandgap reference voltage VREF is given by equation 1.4:
                              V          REF                =                                            V                              BE                ⁢                                                                  ⁢                1                                      +                                                            Δ                  ⁢                                                                          ⁢                                      V                    BE                                                                    R                  1                                            ·                              R                2                                              =                                    V                              BE                ⁢                                                                  ⁢                1                                      +                                                            R                  2                                                  R                  1                                            ⁢                              k                q                            ⁢                                                ln                  ⁡                                      (                    m                    )                                                  ·                T                                                                        Equation        ⁢                                  ⁢        1.4            
Adjustment of the ratio of first and second resistors R1 and R2 compensates the temperature coefficient of bandgap reference voltage VREF. The objective is to render bandgap reference voltage VREF with zero temperature coefficient, i.e., independent of temperature fluctuations, so that it can be a temperature independent reference voltage. Temperature independent bandgap reference voltage VREF is an input to conventional sigma delta modulator 140.
In conventional sigma delta ADC 130, conventional sigma delta modulator 140 and conventional digital filter 150 use PTAT voltage ΔVBE and temperature independent bandgap reference voltage VREF to generate a digital voltage ready to be converted to temperature by conventional voltage to temperature converter 160. The output of conventional voltage to temperature converter 160 is conventional temperature measurement CTemp.
There are a number of problems with conventional temperature detection systems such as conventional temperature detection system 100. Generally, it is difficult to manufacture a highly accurate Silicon PN junction sensor because the PTAT voltage ΔVBE is only tens of milliVolts (mV), with only a few tenths mV change for each degree Kelvin of temperature variation, there may be mismatch between first and second FETs M1P, M2P, mismatch between first and second resistors R1 and R2, mismatch from the current density m between first and second BJTs Q1P, Q2P, all of which may cause several degrees of error in PTAT voltage ΔVBE. Generally, these and other problems require substantial post-processing (e.g. trimming, calibration circuitry) to correct conventional temperature output CTemp. In greater detail, eight specific problems are addressed below.
First, equations 1.1-1.4 are for ideal behavior of conventional stimulator 110 and conventional temp sensor 120. However, operation of their components is unlikely to be ideal. This may induce an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Second, the actual current density ratio between first and second BJTs Q1P, Q2P may not be exactly the current density ratio m that the design and equations 1.1-1.4 are based on. This may induce an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Third, there may be a Beta β (i.e. Ic/Ib) mismatch for first and second BJTs Q1P, Q2P at different current densities. In standard CMOS fabrication processes, the only available options for BJTs is lateral PNP, which have lower current gain. Equations 1.1-1.4 are for collector current, but they need to be modified for emitter current in accordance with Equations 1.5-1.7 below:
                                              ⁢                              V                          BE              ⁢                                                          ⁢              1                                =                                    kT              q                        ⁢                          ln              ⁡                              (                                                      I                                          E                      ⁢                                                                                          ⁢                      1                                                                                                  (                                              1                        +                                                  β                                                      F                            ⁢                                                                                                                  ⁢                            1                                                                                              )                                        ⁢                                          I                                              s                        ⁢                                                                                                  ⁢                        1                                                                                            )                                                                        Equation        ⁢                                  ⁢        1.5                                                          ⁢                              V                          BE              ⁢                                                          ⁢              2                                =                                    kT              q                        ⁢                          ln              ⁡                              (                                                      I                                          E                      ⁢                                                                                          ⁢                      2                                                                                                  (                                              1                        +                                                  β                                                      F                            ⁢                                                                                                                  ⁢                            2                                                                                              )                                        ⁢                                          I                                              s                        ⁢                                                                                                  ⁢                        2                                                                                            )                                                                        Equation        ⁢                                  ⁢        1.6                                          Δ          ⁢                                          ⁢                      V            BE                          =                                            V                              BE                ⁢                                                                  ⁢                1                                      -                          V                              BE                ⁢                                                                  ⁢                2                                              =                                                    kT                q                            ⁢                              ln                ⁡                                  (                                                                                                              I                                                      E                            ⁢                                                                                                                  ⁢                            1                                                                          ⁢                                                  I                                                      s                            ⁢                                                                                                                  ⁢                            2                                                                                                                                                I                                                      E                            ⁢                                                                                                                  ⁢                            2                                                                          ⁢                                                  I                                                      s                            ⁢                                                                                                                  ⁢                            1                                                                                                                ⁢                                                                  (                                                  1                          +                                                      β                                                          F                              ⁢                                                                                                                          ⁢                              2                                                                                                      )                                                                    (                                                  1                          +                                                      β                                                          F                              ⁢                                                                                                                          ⁢                              1                                                                                                      )                                                                              )                                                      =                                          k                q                            ⁢                                                ln                  ⁡                                      (                                          m                      ⁢                                                                        (                                                      1                            +                                                          β                                                              F                                ⁢                                                                                                                                  ⁢                                2                                                                                                              )                                                                          (                                                      1                            +                                                          β                                                              F                                ⁢                                                                                                                                  ⁢                                1                                                                                                              )                                                                                      )                                                  ·                T                                                                        Equation        ⁢                                  ⁢        1.7            Equations 1.4-1.7 show that current density cannot be too large and that biasing points must be carefully selected to try to make forward gain nearly constant in order to render Beta factors negligible. Of course this is difficult to accomplish and adjustments may be necessary.
Fourth, mismatch in the current mirror created by first and second FETs M1P, M2P may cause the bias currents in first and second BJTs Q1P, Q2P to be different. Such an error would mean current density m is something other than m, which would result in an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Fifth, parasitic resistance exists between terminals and components. For examples, a parasitic resistance RC is in series with the collectors of first and second BJTs Q1P, Q2P. The voltage detected always includes voltage across parasitic resistance. This may induce an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Sixth, an offset voltage may exist due to op amp 125. This may induce an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Seventh, analog to digital conversion may introduce errors and be reflected as an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
Eighth, an error in bandgap reference voltage VREF will be reflected as an error in PTAT voltage ΔVBE. Accordingly adjustments may be necessary.
These problems are typical in conventional temperature detection systems. With so many adjustments necessary to compensate for so many so many sources of errors, such as non-ideal components and non-ideal performance, it is inevitable that conventional temperature output CTemp will contain errors. Thus, there is a need for a temperature detection technique that eliminates or reduces the impact of common sources of error.